Reduction of static interference in radioreceiving stations



A. M. CURTIS.

TERFERENCE INRADIOR ECEIVING STATION S. APPLICATION FILED OCT. 8, 1917. I

REDUCTION OF STATIC IN VFW-H HH'Hy 1" W mm w /n wnlor:

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, Patented Mar. 15,1921.

UNITED STATES PATENT OFFICE.

AUSTEN M. CURTIS,

OF BROOKLYN, NEW YORK, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

REDUCTION OF STATIC INTERFERENCE IN RADIORECEIVING STATIONS.

Specification of Letters Patent. Patented Bla r. 15, 1921.

Application filed October 8, 1917. Serial No. 195,371.

To all whom it may concern:

Be it known that I, AUsTEN M. CURTIS, a citizen of the United States, residing at Brooklyn, in the county of Kings and State of New York, have invented certain new and useful Improvements in Reduction of Static Interference in Radioreceiving Stations, of which the following is a full, clear, concise. and exact description.

p The present invention relates to a circuit comprising a novel form of wave filter and to its use in radio receiving systems for the purpose of reducing the effect of static or atmospheric disturbances produced in the signaling system when signals are being received.

The invention makes use of a circuit comprising a wave filter constituted by resistance elements and elements of one kind of reactance and substantially free from reactance of the other kind. For example, the filter may be made up of resistance and capacit elements, the filter being substantially free from inductance, or it may be formed by resistance and inductance elements and may be substantially free from capacity. Any fortuitous inductances or capacities are so extremely small in magnitude that the frequencies of the natural oscillations of the filter are very high in comparison with the frequencies of the oscillations which the filter efficiently transmits. To prevent reaction between the filter sections, unidirectionally transmitting elements,

such as the well-known three-element vacuum tube amplifier, may be inserted between adjacent filter sections.

A filter which transmits currents of all frequencies below a fixed cut-off frequency and suppresses those of higher frequency is termed a low pass filter. One which transmits currents of all frequencies above a fixed cut-off frequency and suppresses those of lower frequencies is termed a high pass filter. A low pass filter having a given cutoff frequency and a high pass filter of lower cut-off frequency connected in se'm'atim constitute a band filter which will pass currents of a band of frequencies extending between the two cut-off frequencies and will extinguish currents of frequency outside this band. Filters of this type are disclosed and claimed per 86 in the copending applications of A. M. Curtis, Serial No. 315,842, filed August 7, 1919, and Serial No. 316,359, filed August 9, 1919.

According to the present invention, a filter of the character described'substantially free from one kind of reactance and all the natural oscillation frequencies of which are very high and outside the frequency transmission band of the filter, is used in a radio receiving system to discriminate between static energy and sustained oscillations which it is desired to receive. A particular advantage of sucha system is that the energy of a static impulse which is impressed on the filter is largely dissipated in producing natural oscillations of such high frequencies that the filter will not transmit the oscillations, and hence this energy does not affect the signal receiving instruments.

The effect of static is furthermore reduced according to the present invention in the following way: the antenna is detuned, that is, tuned to a frequency different from the signal frequencies. This means that the signaling frequenc will not be received by the antenna as e ciently as either (1) damped oscillations produced in the antenna by a static impulse, or (2) static wave trains hav- I ing frequencies in the neighborhood of the frequencies for which the antenna is tuned. This decrease in the efficiency of"the reception of the signaling frequencies is more than off-set by providing a band filter such as described, and an amplifier, the filter and amplifier serving toselectively amplify currents of the signaling frequency to a much greater extent than the different frequency currents produced by the static. If the antenna were tuned to the signaling frequency then static would set up currents of the same frequency as the signaling currents, and'no frequency discrimination between static and signal would be possible. According to the presentinvention, however, the antenna is detuned so that such energy of the static as is converted to natural oscillations by the detuned antenna will be re- 100.

. and Fig. 2 illustrate 7 shown in block A. In advance of frequency which is different from the signaling frequency. This makes it possible to select currents of the signaling frequency and to amplify them until the static is relatively insignificant.

This will be further described in connection with the drawings, in which Figure 1 low pass and high pass filters respectively, employing capacity and resistance elements. Fig. 3 illustrates diagrammatically the filters in Figs. 1 and 2 in a simple form. Figs. 4 and 5 illustrate low pass and high pass filters respectively employing resistance and inductance elements. Fig. 6 shows diagrammatically the filters in Figs. 4 and 5 in a simple form. Fig. 7 illustrates how the filters of Figs. 1 and 2 may be combined to provide a band filter. Fig.

ceived at a 8 illustrates a circuit, comprising filters of the character described which may be emplqyed in a static reducing system.

eferring to Fig. 1 the line 1 is adapted to receive currents of various frequencies, the low frequency components of which are to be selectively transmitted and amplified. The, block F represents a filter and amplifier section, a plurality of which may be used as represented by the other blocks F all of these being connected in tandem. The block F comprises a vacuum tube 2 of the audion type having a high resistance 3, in shunt to its input circuit, a battery 4 which assigns a negative potential to the grid 5, and a source 6 which supplies the space current for the tube through the high non-inductive resistance 7. The filament 8 of the tube is grounded as shown at 9. p In shunt to the output circuit of the tube 2 is a circuit which includes a high non-inductive resistance 10 and a capacity 11 which is grounded at 12. The next block F is connectedv in shunt to the condenser 11- as shown, and all succeeding filter sections are similarly connected to their respective preceding sections.

ployed in each of sentially unidirectionally conducting so that reaction between the filter sections is prevented. The current delivered by the last filter section F, may be amplified by an ordinary amplifier of the audion type, as

the amplifier in block A is provided a condenser 13 which prevents the battery in the output circuit of the preceding tube in the series from supplying unidirectional current to the grid-cathode path of the amplifier A.

he condenser 14 in block F performsa similar function.

In F ig. 2 the filter section F 2 is similar to F except that the positions of the resistance 10 and the capacity 11 have been interchanged. In Fig. 2 the second filter section F is in shunt to the resistance 15, which together with the condenser 16 forms The vacuum tube emthe filter sections is es- .hlgh frequencies and operates a shunt path across the cathode and anode of the tube 17. In this case also a plurality of filter sections F 2, and an amplifier A may be employed.

he operation of the filters shown in Figs. 1 and 2 may be more readily explained by referring to Fig. 3, in which the line 18 is adapted to receive currents of various frequencies, some only of which are to be efliciently transmitted. Connected to the line 18 are the resistance 19 and capacity 20. It will be apparent that currents of very low frequencies find a path of very high impedance through the condenser 20, so that the voltage across the condenser 20 for these low frequency currents is relatively high, the voltage across the resistance 19 being relatively low since the impedance of a condenser at low frequencies is relatively high. If a voltage responsive device such as a voltmeter 21 is connected across the condenser 20 it will register a comparatively high voltage for low frequency currents which are received by the line 18. High frequency c'urrents,however, find a path of low impedance through the condenser 20, so that the voltage drop for high frequency currents across condenser 20 as observed by the p voltmeter 21 will be comparatively low while the drop across the-resistance 19 will be comparatively high, due to the fact that the impedance of a condenser at high frequencies is relatively low. Of the various frequencies which are received by the line 18 the lowest frequencies produce the greatest effect upon the voltmeter 21, so that the effect is to suppress the high frequencies, and hence this is a filter of the low pass type. The connection-of the voltmeter 21 across the condenser 20 is entirely analogous to the connection of the second filter F of Fig. 1 across the condenser 11. In Fig. 1, however, a plurality of such sections are employed, a unidirectional tube being located between adjacent sections to prevent reaction. Furthermore, the output of the last filter section. is connected to an amplifier tube to bring the amplitude of the selected frequency or frequencies up to the desired value.

With regard to Fig. 8 it is apparent that low frequency currents produce a low voltage across resistance 19 while high frequencies give rise to a high voltage. This serves to suppress low frequencies and accentuate as a high pass filter as is the case in Fig. 2. In Fig. 4 a low pass filter is provided by reason of the fact that the inductance 22 is connected in the output circuit of the tube 23. A condenser 24 is here employed to prevent the battery 25 from impressing a unidirectional E. M. F. upon the grid filament circuit of the succeeding tube.

The showing in Fig. 5 is similar to that of Fig. 4, except that the positions of the inductance 22 and resistance 26 in Fig. 4 are interchanged, the second filter section F being connected in shunt to the induc-- tance 27 The operation of the filters shown in Figs. 4 and 5 will be described in connection with Fig. 6, in which the line 28 is adapted to receive currents of various frequencies, some of which are to be transmitted .more efiiciently than others." The line 28 includes a resistance 29 and inductance 30. 7 At low frequencies the voltage across the inductance 30 as observed by the voltmeter 31 is very low, while at high frequencies it will be high, and the converse will be true with respect to the voltage across the resistance 29. It will be apparent, then, that Fig. 4 discloses a low pass filter and Fig. 5 a high pass filter.

In Fig. 7 the filters shown in Figs. 1 and 2 have been combined to provide a filter for transmitting only a band of frequencies, the high pass filter serving to suppress the lower frequencies up to a certain value and the low pass filter suppressing all frequencies beyond a given value. In this case the condenser 16 prevents the battery 6 from impressing a unidirectional E. M. F. upon the grid cathode circuit of the tube 17 The filters shown in Figs. 1, 2 and 7 are substantially free from inductance, and they accordingly have a very high frequency of natural oscillation.

The general principles of design for vacuum tube circuits apply to the circuits of this invention, due regard bein given the fact that stability and selectivity are of more importance in attaining the desired object than is absolute efliciency of transmission considered from a stand oint of energy loss. For efiicient transfer 0 energy, as is well known, the impedance of the circuit into which a vauum tube works as measured at the output terminals of the tube for currents of the frequencies to be transmitted should be of the same order of magnitude as the internal impedance of the tube itself. For stability the resistance 7 in the space current path should be high so that the total impedance of the cathode-anode circuit of the vacuum tube does not change very much Within the range of frequencies of the oscillations which it is desired to transmit. Since the selectivity is a function of the relative impedance of the resistances and reactances of the filter for desired and undesired oscillations, the magnitudes of these filter element impedances may be widely varied according to the degree of selectivity desired and the energy loss of the desired signaling currents for which it is possible to compensate by amplification.

In the radio system shown in Fig. 8 three filter sections F and an amplifier A, and three filter sections F and an amplifier A are all connected in tandem and located between the antenna 32 and the detectors 33 and 34. The antenna 32 is detuned, that is, tuned to a frequency difierent from the sig naling frequency, for instance the antenna may be tunedto 15,000 cycles if the signaling frequency is 30,000 cycles The band filter provided by the sections F and F transmits selectively a narrow band of frequencies corresponding tothe signaling frequency which may be 30,000 cycles. By means of the switch 35 the wave filter system may be connected either directly to its detector which may be of the audion type, as 33, or it may be connected to a circuit 36, tuned to the signaling frequency,-which is in turn connected to a detector 34. The detectors 33 and 34 are provided with the usual-receivers 37 or other signal translatingi means.

he operation of the receiving system shown in Fig. 8 is as follows: Assume that signals are being received at a frequency of 30,000 cycles and that the impedance of the antenna system adjusted so that when it is tuned to 15,000 cycles the current strength of the signals is only 'of what it would be if the antenna were tuned to 30,000 cycles. In detuning this amount we have increased the inductance in the antenna system four times. Imagine an impulse taking place in the neighboring ether and affecting the antenna. The maximum value of the damped oscillation or impulse produced is inversely proportional to the inductance of the antenna, and consequently would be only one quarter of the value it would have if the antenna were tuned to 30,000 cycles. By this detuning we have, therefore, increased the static two and onehalf times in relation to the signals.

Now consider thatthere is connected to the antenna system some means which at 30,000 cycles amplifies thirty times as much as it does at 15,000 cycles. This selective amplification is made possible by thefilters shown herein: On the input circuit of the filter the damped oscillations produced by the static impulse are two and one-half times as strong as the signals, if we assume that the static and signals are of the same original strength, but the signals will be .amplified thirty times as much as the static. Consequently in the output circuit of the vacuum tube filter the signals will be twelve times as strong as the static. Both the static and signals are in the same form in which they would be obtained from the antenna directly, since all of the natural periods of the filter system are almost infinitely high in relation to the-signaling frequencies.

efiiciently than the signals and if the same ratio holds for the reception of signals and damped static, the static of this type would then'be ten times as strong as the signals. As pointed out above this is due to the fact that the antenna is adjusted so that it recelves currents of 15000 cycles ten times as efficiently as it receives currents of 30000 cycles. We then supply this current to the filter and amplifier and amplify the signals of frequency 30000 thirty times as much as the static of frequency 15000. The net result is that at the output circuit we obtain signals which are three times as strong as the static if they were originally of the same strength. In either case a suppression of the static is effected.

What is claimed is:

1. In a signaling system the combination of a detuned antenna, a frequency selective amplifying means associated therewith, and means including a band filter all the natural oscillation frequencies of which are outside the frequency transmission band of sald filter associated with said amplifier for translating the signals.

2. The combination of a receiving system comprising a detector, a wave filter in advance of said detector, all of the natural oscillation frequencies of said wave filter being outside of the range of frequencies efficiently transmitted by said filter.

3. In a radio receiving system, a wave filter comprising resistance elements and elements of only one kind of reactance, each of said resistance elements being directly connected to a reactance element, means whereby each of said resistance elements and its directly connected reactance element are subjected to respectively different elect-romotive forces and a detector connected to said filter.

4:. A wave filter comprising series resistance and shunt reactance impedance elements and said filten being substantially free from inductance, and detecting means responsive to the potential across one of said elements.

5. A wave filter comprising both shunt resistance and series condenser elements and a vacuum tube detector responsive to the potential across said resistance element.

6. A transmission system comprising a unidirectionally conducting element and a detector and a band filter between said element and detector, said band filter having a natural period of oscillation which is outreactance and another of said sections comprising series capacity reactance and shunt resistance-' 8. A wave filter havinga plurality of sections each comprising a plurality of resistance and capacity elements and substantially no inductance, a unidirectionally conducting device between adjacent sections of said filter and a detector connected to the last section of said filter.

9. The combination of a unidirectionally conducting element and detector, a connecting line there-between, said line comprising series resistance and series condenser elements and also shunt resistance and shunt capacity elements, a source of energy for said unidirectionally conducting element, one of said condensers preventing current from said source from being'supplied to said detector.

10. A signaling system comprising a unidirectional receiving conductor, a detector, and a wave filter between said conductor and said detector, said filter comprising resistance and capacity reactance elements and being substantially free from inductance.

11. A signaling system comprising a receiving conductor, a detector, and a wave filter between said conductor and said detector, said filter comprising a plurality of sections each comprising resistance and capacity reactance elements, and a substantially unidirectionally conducting device between adjacent sections of said filter.

12. A receiving system comprising a receiving conductor, a. translating devlce and means including a wave filter connecting said receiving conductor to said device, said filter having a transmission range including a band of frequencies and excluding substantially all frequencies outside said band and having no natural oscillation frequency within said band.

13. A receiving system for discriminating between sustained oscillations of a given frequency and disturbin energy, comprising a receiving con uctor responding strongly to oscillations of frequency widely different from said given frequency, a translating device and a network comprising resistance and only one kind of reactance connecting said conductor network having a transmission frequency band excluding frequencies of all the natural osqikllations of said network.

receiving system for discriminating between periodic sustained energy of a to said device, said a I given frequency and disturbing energy, comprising a receiving conductor tuned to a frequency different from said given frequency, a translating device and a network connecting said conductor to said device, said network comprising a band filter for transmitting energy of a band of frequencies including said given frequency, and excluding the frequencies of all the natural oscillations of said network. 10 In Witness whereof, I hereunto subscribe my name this 2nd day of October A. D.,

AUSTEN M. CURTIS. 

